Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. In one embodiment, an ion source is used. This ion source may include a plasma chamber in which source gasses are ionized. The ions from these source gasses may be extracted through an aperture in the plasma chamber, using one or more electrodes. These extracted ions are directed toward a workpiece, where they are implanted in the workpiece to form the solar cell.
In an effort to improve process efficiency and lower cost, in some embodiments, the ions extracted from the ion source are accelerated directly toward the workpiece, without any mass analysis. In other words, the ions that are generated in the ion source are accelerated and implanted directly into the workpiece. A mass analyzer is used to remove undesired species from the ion beam. Removal of the mass analyzer implies that all ions extracted from the ion source will be implanted in the workpiece. Consequently, undesired ions, which may also be generated within the ion source, are then implanted in the workpiece.
This phenomenon may be most pronounced when the source gas is a halogen-based compound, such as a fluoride. Fluorine ions and neutrals (metastable or excited) may react with the inner surfaces of the ion source, releasing unwanted ions and neutrals, such as silicon, fluorine, oxygen, carbon, and aluminum and heavy metals present as impurity elements.
Therefore, a method that improves beam quality, particularly for embodiments in which halogen based boron-containing source gasses are employed, would be beneficial.